High-pressure discharge lamps usually have electrodes made of tungsten, which is doped with an electron emitting material. The usual doping material is thorium oxide, that is ThO.sub.2, see, for example, the referenced U.S. application Ser. No. 07/215,829, filed July 6, 1988, now U.S. Pat. No. 4,906,895, Pabst et al. It has been found that granules or slugs, due to meltback can occur at the end portion of the electrode in types of lamps which operate at very high alternating current levels, for example in the order of between about 20 to 60 amperes. Such slugs, pearls, or granules may occur already comparatively shortly after the lamp operates that is, between about 20 to 100 hours of operation. These granules lead to undesired premature blackening of the inner wall of the lamp bulb. If the size of the granules or pearls exceeds a predetermined value, it cools the remainder of the electrode tip and interferes with the migration of emitting material and with the possibility of the discharge arc to find a new starting point. Overheating of large melt-back granules lead to vaporization of tungsten, which further blackens the lamp bulb.
The foregoing phenomenon occurs only when the current levels are high and the operating pressures are high, which result in constriction of the arc and reduction of the size of the spot or location where the arc forms, thereby increasing the temperature at the said arc spot.
It has previously been proposed to use lanthanum as an electron emitter substance, see, for example, German Patent Disclosure Document DE-OS 31 19 747. According to this disclosure, lanthanum oxide, La.sub.2 O.sub.3 is used as a component for the emitter paste applied to the filament of a fluorescent lamp. U.S. Pat. No. 4,019,081, Buxbaum et al describes the use of La.sub.2 O.sub.3 applied to the cathode of vacuum tubes. Neither application of lanthanum oxide, La.sub.2 O.sub.3 is concerned with the formation of melt-back granules or slugs which may cause blackening of the wall of the discharge vessel; in either case, the current flowing through the cathodes which use the lanthanum oxide are low with respect to the currents of high pressure, high power discharge lamps; likewise, the operating pressures are low.